This application discloses an invention which is related, generally and in various aspects, to a system and method for measuring geometric change in a subterranean structure. Although the subterranean structure can be any type of subterranean structure (e.g., a mine, a tunnel, a cave, a bunker, a conduit, etc.), for purposes of simplicity, the subterranean structure will be described hereinafter in the context of a mine.
One type of geometric change is convergence, which is also called squeezing or closure. Convergence is a natural process in underground mining that can be defined as the measurable closure within a cross-section of an excavated space. Closure is a natural action of the remaining rock under pressure that is modulated by its mechanical properties and features. The presence of squeezing indicates a level of instability that can lead to large scale deformation, and in some cases, complete loss of structural integrity and collapse within the area of concern.
Since squeezing is a geometric contraction of a cross-section of an excavated space, it can be measured via direct geometric measurements within the cross-section of the excavated space. In some instances, squeezing is deemed to have occurred if the geometric measurement is reduced by 10 centimeters over the operational lifetime of the given excavated space. Alternative definitions indicate the presence of convergence in areas of the mine that exhibit stress exceeding 2%. Stress, in this case, is expressed as engineering strain, which is the ratio of the applied load to the original cross-sectional area.
Another type of geometric change is subsidence, which is a shifting of layers of the earth's surface downwards with respect to gravity. It is usually manifested as a pit or trough in form. With respect to underground mining, subsidence is usually caused by the collapse of underground structure beneath the subsiding layer. This phenomenon can be planned or unplanned. In block cave mining the formation of a pit on the surface of the mine is part of the mine development process. In other cases, the unplanned or unexpected collapse of underground structures can lead to the subsidence in the layers of the earth above the collapse. Subsidence can occur suddenly and catastrophically as rapid convergence, or it can develop more slowly over time. In complex mines, such as multi-layer block cave mines, production in a lower layer may cause subsidence in the layer adjacent and above it within the mine.
Underground mines are intricate structures that can move in complex ways that cannot be described completely by concepts such as convergence or subsidence. For example, convergence and subsidence could be happening within the same general area of the mine where the measurements are taking place. Isolated measurements in the cross-section will do a poor job of discriminating between the components of the measurement that can be attributed to convergence and the components of the measurements that should be attributed to subsidence. In the worst case, an insufficient program of measurements could lead to complete misclassification of the phenomena underlying the measured changes.
More generalized forms of mine movement are not constrained to linear combinations of subsidence and convergence. More complex forms of ground movement that involve mechanical concepts like torsion of a non-uniform medium under stress and other generalized forms of distortion are present to varying degrees within underground excavations. The more complex the form of motion or distortion manifested by the underlying phenomena, the less likely simple cross-section measurements are to be able to identify, classify, or even measure the underlying geotechnical issues manifested as movement within the cross-section.
Current best practice for detecting and measuring ground movement in underground mining is the use of an extensometer. An example of an extensometer is shown in FIG. 1. An extensometer is an engineering measurement device capable of making precision measurements of length that are useful for stress-strain analysis. Extensometers can come in contact or non-contact (laser and video, for example) varieties. In underground mining, it is common practice to make repeated extensometer measurements from within a cross-section of an area of concern within a mine. By making repeated measurements from the same area at different times, incremental measurements of ground movement are obtained. Geo-technical engineers analyze time-series of these displacement measurements in order to improve ground support and ideally prevent any unwanted collapse of an area of the mine.
Extensometers are often custom designed to provide a precise measurement for the type of movement that is assumed to be present within an area. If the actual ground movement differs from the assumed model, then a precision measurement is made and applied to the wrong ground movement model. At best, this practice leads to the incurring of additional and potentially unnecessary ground support that increases the overall cost to a mining operation. In the worst case, significant ground movement events are misclassified inviting catastrophe.
U.S. Pat. No. 7,069,124 to Whittaker et al., titled “Robotic Modeling of Voids”, describes a system for the mapping and inspection of underground voids in two or three dimensions. A preferred embodiment of the invention disclosed therein includes a method for mapping subterranean voids such as, for example, caves, tunnels, bunkers and conduits. The method includes the following subprocesses: preprocessing, ingress/deployment, void modeling, mapping and navigation, exploration, conveying payloads other than void modeling sensors, stowing/egress and post processing.
International Application Publication No. WO 2013170348 to Tesman, Inc., titled “Mapping of Mining Excavations”, describes an apparatus that includes at least two cameras and is suitable for installation on a mine excavation vehicle. The apparatus and methods disclosed therein concern the generation of three-dimensional (3D) models, or digital representations, of mining excavations. An embodiment of the invention disclosed therein includes an installation of the apparatus on a mining vehicle as well as use of the apparatus in the act of controlling at least one operation of the vehicle.
U.S. Pat. No. 8,164,473 to Roy, Jr., titled “Mine Roof Monitoring Apparatus”, describes a contact measurement apparatus for measuring subsidence in underground mines. An embodiment of the invention shown in FIG. 1 is a first contact member in contact with a roof of the mine and a second contact member in contact with a floor of the floor. A measurement device is attached to one of the contact members to enable the measurement device to measure any change of the separation between the roof and the floor. In another embodiment, the contact members are capable of expanding or contracting to enable the measuring device to continue to make measurements after subsidence has occurred.
U.S. Pat. No. 4,514,905 to Lutzens, titled “Convergence Extensometer for Measuring Mine Roof Subsidence”, describes a contact extensometer device which measures changes in the distance between a mine roof and a mine floor to determine mine roof subsidence. An embodiment of the contact extensometer device includes a pair of telescoping members secured via a spring in tension and an indicator that is capable of measuring the movement of the telescoping members. One of the members is attached to roof of the mine and is in vertical alignment with the other member that is attached to the floor of the mine. The indicator includes a pair of flanges, or lugs, which can be attached to each of the telescoping members fixed to the mine roof and floor.
U.S. Pat. No. 3,894,427 to Schuermann et al., titled “Device for Measuring Changes in Converging Rock Formations in a Mining Cavity”, describes a device for monitoring and measuring changes in converging rock formations within a mining cavity. The device includes an elongated tube affixed via adhesive into bore holes drilled into opposite walls of the mining cavity. The tube contain a series of strain gages that are internally recessed and axially spaced at intervals along the tube. The invention disclosed therein enables the detection of changes within a rock formation that is surrounding the mining cavity with a single anchor. Furthermore, measurements of convergence are obtained by mounting two coaxial anchors on opposite sides of the mining cavity.
China Patent Application No. 103791802, titled “Underground Tunnel Two-Side Convergence and Deformation Electronic Measurement Device and Method”, describes a device which includes a fixed two-sided displacement detecting device, a deformation sensing device and a data collection device.
Canadian Patent Application No. 2019894, titled “Mine Convergence Monitor”, describes an apparatus which includes an optical head which is mounted to a fixed point on the mine wall, a target reflector which is mounted on another portion of the mine wall and facing the optical head, and a time domain reflectometer for measuring the time taken to transmit a beam of light between the optical head and the target reflector. The apparatus also includes means for deriving the distance between the target reflector and the optical head from the previously mentioned transmission time. It also discloses a means for determining convergence by comparison and subtraction of a prior measurement to a current measurement to the same optical head and target reflector as mounted in the same relative positions on the wall.